This application claims priority of European Patent Application No. 98460058.5, which was filed on Dec. 15, 1998.
1. Field of the Invention
The field of the invention is that of methods for the transmission of telephone calls in multiple bit-rate transmission channels.
The invention can be applied especially to the case of multiple bit-rate links set up between a central station and the remote peripheral stations of a network in a star or hub-and-spoke configuration.
2. Description of the Related Art
A network of this kind corresponds for example to  less than  less than rural greater than  greater than  telephony infrastructures, typical in regions that are under-equipped in wire or cabled networks and/or have a low density of potential subscribers.
It can be applied especially when a terminal involved in a communication using a network of this kind may require resources with different bit-rates depending on whether it is seeking, for example, to set up either a simple voice call or a transfer of data of the type requiring action by a modem (for example for a connection on an  less than  less than internet greater than  greater than  type open network. In this case, indeed, a voice call will require a lower transmission bit rate than a data transfer.
A first known approach consists in routinely providing a bit rate that is capable of coping with the highest resource requirement. However, this approach means under-utilization of the transmission channel since, in the case of simple voice calls for example, the allocated transmission bit rate will be far greater than needed.
A second approach consists of the use of a multiple bit-rate channel to transmit this type of call. However, those skilled in the art are dissuaded from using this technique because of the complexity of the procedures for allocating existing bit rates, which are not compatible with the constraints of communications involving modems, especially in the vital initial stages of setting up calls and of inter-modem negotiation.
Indeed, a change in the bit rate of the line allocated as a function of need implies the implementation of bit-rate switching procedures, to increase or reduce the bit rate allocated to the demand. However, a classic switching of the transmission channel entails the risk that the bit-rate switching is not exactly simultaneous and synchronized in both directions of transmission. The disturbance resulting therefrom is indeed likely to abort the procedure for setting up communications between the modems, and this risk cannot be tolerated in the context of a switching service which is expected to show reasonable reliability and efficiency with a success rate close to 100% when the communications are set up.
The call procedure between two modems, one of them being with a multiple bit-rate telephone subscriber (calling or called subscriber), can be subdivided into four phases:
making the 32 kbit/s call;
connecting the two modems;
procedure for negotiating the speed of transmission between the two modems;
transfer of the data elements.
In accordance with recommendation V25, the modem connection phase can be subdivided as follows:
the calling modem, after getting linked up and dialling the number, sends an uninterrupted call tone to the called modem;
the called modem takes the call, gets connected to the line and, after a silence, sends the response tone constituted by a continuous signal at 2100 Hz;
as the case may be, the modems can deactivate network echo cancellers and/or suppressors devices, by making or not making phase reversals at regular intervals (typically every 450xc2x125 ms), as specified in the G164 and G165 recommendations.
the calling modem, after recognizing the answering tone, waits for reception of a silence of 75xc2x125 ms coming from the called modem, and then goes into the negotiation phase. To accelerate the procedure, this calling modem can inform the called modem that it has recognized its reponse by sending it a response tone of the calling modem: in recognizing this signal, the called modem stops sending the response tone and then, after 75xc2x120 ms of silence, goes into the negotiation phase.
In the absence of response signal of the calling modem, the called modem sends the response tone for 3,3 sxc2x10,7 s, then after 75xc2x120 ms of silence goes into the negociation phase.
To enable the modems to work with an optimum bit rate, the switching of the bit rate (namely the changing of the transmission channel) must be ended before the beginning of the procedure of negotiation between the modems.
This means that the following conditions should be met, in assuming the most unfavorable case, namely the setting up of a call with a minimum-duration response signal of the called modem:
ixe2x80x94the response of the called modem should not be interrupted for a duration of more than 55 ms, otherwise the calling modem could go into the negotiation phase;
iixe2x80x94no action (such as a bit-rate switching operation) on the response half-channel of the called modem should take place after the lapse a certain period of time starting from the response tone on the line, beyond which the phase of negotiation is automatically started (655 ms in the case of an operation of neutralization of the echo suppressors only, 1 255 ms in the case of a neutralization of the echo cancellors and suppressors);
iiixe2x80x94any action must also take place after approximately 950 ms, i.e. for the duration of at least two phase reversals, should the modems seek to neutralize both the echo cancellors and the suppressors.
The invention forms part, especially, of this context of constraints that could be contradictory.
Thus, it is a first goal of the invention to put forward the novel problem of the default allocation, at each request for setting up a call, of a minimum transmission bit rate, although this bit rate is paradoxically insufficient for certain types of communication available to the subscriber.
A complementary goal is to provide a call setting-up procedure that makes it possible to optimize resource allocation by the authorizing, at request, of the allocation of a bit rate higher than the bit rate allocated by default.
Another goal of the invention is to provide a bit-rate allocation procedure of this kind that is compatible with the severe constraints dictated by the procedures for setting up calls between two modems.
It is also a goal of the invention to provide a bit-rate allocation procedure that does not require the addition of a piece of information or the processing of a piece of information supplementary to the items of information presently available in standard call-making procedures and that is activated in particular without any advance knowledge of the nature of the transmission requested by the user or any advance knowledge of the transmission bit rate required by his call.
It is a goal of the invention therefore to provide a solution that meets at least the case where the modems seek to neutralize both the echo cancellers and the echo suppressors during the connection phase when the bit-range switching according to the invention comes into play.
These different goals as well as others that shall appear hereinafter are achieved according to the invention by means of a call setting-up procedure with bit-rate allocation in a multiple bit-rate transmission channel enabling the routing of a call between a central module and a remote module, of the type comprising at least one first transmission channel at a primary bit rate and at least one second transmission channel at a secondary bit rate, a procedure in which said first and second transmission channels are two-way channels, namely channels constituted by a first transmission half-channel in a first direction of communication and a second transmission half-channel in a second direction of communication, a procedure wherein said call setting-up procedure comprises the following successive phases:
a phase for setting up a call, by default, on a primary bit-rate transmission channel,
a phase of listening and searching for a signal indicating a secondary bit-rate requirement and, assuming the detection of a signal indicating a secondary bit-rate requirement,
a phase of bit-rate switching from said first primary bit-rate transmission channel to a second secondary bit-rate transmission channel, said switching being done at different points in time for said first transmission half-channel and for said second transmission half-channel.
In other words, the general principle of the invention thus relies on the passage of an ongoing call from a transmission of data on a first channel at a bit rate with a first value (called a primary bit rate) to a transmission of data on a second channel at a bit rate with a second value (called a secondary bit rate). To obtain this passage, certain conditions have to be fulfilled to bring about the sending, for example by the remote module, of a signal enabling the identification of a secondary bit-rate requirement. It is the detection, for example by the central module, of this signal indicating a secondary bit-rate requirement that prompts the activation of the bit-rate switching phase.
Hereinafter, in the present description of the patent, the term ( less than  less than half-channel  greater than  greater than  is understood to mean a one-way channel, namely a channel on which the central module receives or sends data from or to a remote module at a given bit rate.
It will be noted that the switching phase takes account especially of the capacities offered by the transmission network. Consequently, the switching phase may be adapted to a large number of configurations of the transmission network. An adaptability of this kind therefore provides great flexibility with respect to the implementation of the method of the invention.
In a preferred mode of operation of the invention in which the central module and the remote module work together with at least one communications modem, said signal indicating a secondary bit rate requirement is sent by one of said modems.
Advantageously, said procedure comprises a complementary phase for the connection of said modems, during which:
the calling modem sends a call tone to the called modem, then;
the called modem sends a response tone to the calling modem during a predetermined and uninterrupted period of time, at the end of which there is a phase of negotiation between said modems, said switching phase comprising the following steps:
a step of activation of a first time lag starting from said detection of a signal indicating a secondary bit-rate requirement,
a first step for the switching of the call, for the half-channel corresponding to the direction of reception by the remote module, from the primary bit-rate transmission half-channel to the secondary bit-rate transmission half-channel,
and, at the end of the first time lag, a second step for the switching of the call, for the half-channel corresponding to the direction of sending by the remote module, from the primary bit-rate transmission half-channel to the secondary bit-rate transmission half-channel, said first time lag having a duration smaller than said predetermined and uninterrupted time period, at the end of which there is a phase of negotiation between said modems.
In other words, the observation of the first time lag activated to pass (from a primary bit-rate data transmission channel) to a secondary bit-rate data transmission channel does not disturb the normal operation of the data exchanges between the models when communication is in progress.
Preferably, with said procedure providing for a step of implementation of a function requiring a predetermined implementation time, said first time lag has a duration greater than said predetermined implementation time.
A duration such as this of the first lime lag enables the step to implement a function to be carried out without interrupting it while it is being carried out.
Advantageously, said step for the implementing of a function requiring a predetermined implementation time is a step for the deactivation of network echo suppressors and/or cancellers.
In other words, the duration of the first time lag makes it possible especially to carry out two phase changes to neutralize network echo cancellers.
According to a preferred mode of operation of the invention, in which said first and second steps for the switching of said half-channels imply elementary switching operations between call connection and routing modules, said elementary switching operations imply call interruptions with a duration below a predetermined threshold.
Thus, the modems are prevented from interpreting a disturbance such as this as a signal of passage to the negotiation phase.
Preferably, said first and second switching steps include operations for the Y connections of a secondary bit-rate transmission half-channel in parallel with a primary bit-rate transmission half-channel, prior to a full substitution of said secondary bit-rate transmission half-channel for said primary bit-rate transmission half-channel.
Connection operations of this kind especially prevent any loss of data transmitted during the call.
It will be understood that, for needs of synchronization between the central module and the remote module, connection operations of this kind will take place, in a first stage, in the direction of reception by the remote module and then, in a second stage, in the direction of sending by the remote module.
Advantageously, said procedure furthermore comprises a step for the incrementation of a counter of failures of said procedure in at least one of the situations belonging to the group comprising:
the occupation of all of said transmission channels at a secondary bit rate;
the reception by said central module of a message reporting that it was not possible for said first switching step to be carried out by said remote module.
In one particular mode of operation of the invention, said modems work according to the V25 recommendation.
According to a particular mode of operation of the invention, said signal indicating a secondary bit rate requirement is constituted by a 2100 Hz tone.
Preferably, said procedure meets at least one of the following approximate temporal values:
said predetermined, uninterrupted time period ranges from 400 to 1255 ms;
said first time lag is in the range of about 800 ms approximately;
said function requires a predetermined implementation time necessitating a time of about 950 ms;
said interruptions of calls with a duration below a predetermined threshold are in the range of about 55 ms at most.
Advantageously, said secondary bit rate is higher than said primary bit rate.
Clearly, it is when the primary bit rate allocated by default is a low bit rate, increased only in the event of the detection of a higher secondary bit-rate requirement, that the invention finds a preferred application, providing for an optimizing of the allocation of the bit-rate resources available.
In one particular mode of operation of the invention, said secondary bit rate is equal to 64 kbit/s and said primary bit rate is 32 kbit/s.
According to a particular mode of operation of the invention, the link between said central module and said remote module is an RF link of the type with connection of telephone subscribers by radio concentration.
Preferably, said central module is constituted a central communications station managing the retransmission of the calls, and said remote module is a terminal station.
Advantageously, said call is transmitted according to PCM encoding techniques.
The invention also relates to a switching method of the type implemented especially in the call setting-up procedure as described here above.
The invention also relates to a transmission network implementing the call setting-up procedure as described further above.